1. Field of the Invention
The present invention relates to a group III nitride semiconductor device (hereafter also referred to as a device simply) and, particularly to a fabrication method of a semiconductor laser device using the same material system.
2. Description of the Related Art
A laser device needs to have a resonator consisting of a pair of flat parallel mirror facets for its operation. For example, in the case of the manufacture of a conventional laser device (Fabry-Perot type) using a semiconductor crystal material such as GaAs, the cleavage nature of GaAs crystal i.e., the substrate crystal is utilized for the fabrication of the mirror facets.
In the case of a group III nitride semiconductor device, it is inevitable to perform the epitaxial-growth of the crystal film onto a dissimilar substrate such as sapphire, SiC or the like, because a nitride bulk crystal is extremely expensive to be used in practice although it could be manufactured.
SiC is not frequently used as a substrate for the nitride devices, because SiC substrates are also expensive and a nitride film on the SiC substrate easily cracks due to the difference in thermal expansion coefficient therebetween. Thus sapphire is commonly used as a substrate for the group III nitride semiconductor laser devices. In the case of epitaxial growth of nitrides on a sapphire substrate, a high quality single-crystal film is obtained on a C-face i.e., (0001) plane of sapphire, or on an A-face, i.e., (11{overscore (2)}0) plane (hereafter referred to as (11-20) plane) of sapphire.
The mirror facets may be formed by an etching process such as reactive ion etching (RIE) instead of cleavage, because it is hard to split the sapphire substrate to laser bars in comparison with the GaAs substrate having been used so far for semiconductor laser devices.
Reactive ion etching is mainly used as a method for obtaining the mirror facets of the nitride semiconductor laser on the sapphire substrate at present. However, the resultant device with the mirror facets formed by the reactive ion etching method has a disadvantage that the far-field pattern of its emitted light exhibits multiple spots. The mass-production-type GaN laser with the cleaved mirror facets is studied again in view of overcoming the multiple spots phenomenon in the far field pattern as mentioned above.
It is a matter of course that the cleavage cannot be preferably performed on sapphire in mass production. Therefore, the following method have been used. First, after forming a thick ground layer of GaN film e.g., at approximately 200 xcexcn thickness on a sapphire substrate, the backside of the sapphire substrate of the obtained wafer is ground or lapped to remove the sapphire portion, so that the GaN substrate is obtained. Next, the epitaxial growth of laser structure is preformed on the GaN substrate. From the obtained wafer, laser devices may be fabricated.
However, the conventional method of lapping the back side the sapphire substrate backside as described above requires many steps, and is complicated. As a result, the method invites a very low yield of the group III nitride semiconductor devices. Such a method is not suitable for mass production.
Although sapphire does not have a definite cleavage plane like a Si or GaAs wafer, a C-face sapphire is fairly easily split along its 1{overscore (1)}00) plane (hereafter referred to as (1-100) plane), and also an A-face sapphire can be easily parted along its (1{overscore (1)}02) plane (hereafter referred to as (1-102) plane), so called R-plane, considerably close to the cleavage of ordinary crystal. It is considered that the formation of the mirror facets of nitride semiconductor lasers on a sapphire substrate may be achieved through following methods: First is a method of growing nitride semiconductor layers on a C-face sapphire substrate and then splitting the wafer along (1-100) plane of the sapphire substrate. Second is a method of growing nitride semiconductor layers on an A-face sapphire substrate and then splitting the wafer along (1-102) plane of the sapphire substrate.
As to the first method of mirror facet formation applied to the device grown on a C-face sapphire substrate, there are problems that a sapphire substrate cannot be split unless the substrate is made thin enough by lapping down the backside of the substrate and that it does not have high reproducibility. These problems are caused by the fact that (1-100) plane of sapphire is not an explicit cleavage plane. Since sapphire is very hard crystal, it cannot be split exactly along a line notched on its surface unless it is made thin enough, and the thickness of the sapphire substrate should be reduced to approximately 100 xcexcm in order to obtain mirror facets practical for laser devices. When lapping the backside of a wafer on which a device structure is already formed, the wafer is warped or distorted due to the difference between thermal expansion coefficients of sapphire and nitrides or due to the residual stress caused by lapping process. When the back of a device wafer is lapped, the wafer is thereby apt to fracture during the process. This is very disadvantageous for mass production. The (1-100) plane of sapphire is not a cleavage plane. Therefore, in many cases GaN is split along in a direction slightly deviated from the cleavage plane thereof, the fracture surface consists of many facts of (1-100) planes of GaN, each of which is the cleavage plane, forming a stepwise appearance. The stepwise appearance causes degradation of the reflectivity and perturbation of the wave front of emitted light and, thereby deteriorates the quality of mirror facets for optical resonance of a laser device.
Whereas, the second method of mirror facet forming method applied to the device formed on an A-face sapphire substrate has a problem that the quality of the fracture plane of GaN is not sufficient.
Since the sapphire substrate can be easily split along its cleavage plane (1-102), so called R-plane, it is possible to cleave the sapphire having a thickness of 250 to 350 xcexcm normally used as a substrate. However, as shown in FIG. 1, when forming a laser structure on the A-face of a sapphire substrate and parting sapphire along its R-plane as depicted by the arrow in the figure, fine striations are formed on the side surface of GaN layers. This is caused by the following reason that the laser wafer splits along the R-plane of the sapphire since a major part of the wafer is made of sapphire. The R-plane of sapphire tilts by an angle of 2.40 from (1-100) plane of the grown GaN as shown in FIG. 2, after a propagating crack along the sapphire""s R-plane reaches at the sapphire-GaN interface, the crack still propagates into GaN still along the R-plane of sapphire up to a certain depth. However, GaN tends to crack on its crystallographic cleavage plane (1-100). Therefore, a plurality of (1-100) facets of GaN are formed in such a stepwise manner that the striations appear on the fracture plane of GaN as shown in FIG. 1.
As a result, in the case of the A-face sapphire substrate, the quality of fracture plane is not very good though it is reproducible.
Therefore, an object of the present invention is to provide a group III nitride-semiconductor laser having high-quality mirror facets for a laser structure and a method of fabricating the laser device with high reproducibility.
A fabrication method according to the present invention is a method for producing a nitride semiconductor laser device having crystal layers each made of a group III nitride semiconductor (AxGa1xe2x88x92x)1xe2x88x92YInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), layered in order on a ground layer (Alxxe2x80x2Ga1xe2x88x92xxe2x80x2)1xe2x88x92Yxe2x80x2Inyxe2x80x2N (0xe2x89xa6xxe2x80x2xe2x89xa61, 0xe2x89xa6yxe2x80x2xe2x89xa61), the comprising the steps of:
forming a plurality of crystal layers each made of group III nitride semiconductor on a ground layer formed on a substrate, the crystal layers including an active layer;
applying a light beam from the substrate side toward the interface between the substrate and the ground layer thereby forming the decomposed-matter area of a nitride semiconductor;
separating the ground layer with the crystal layers thereon from the substrate along the decomposed-matter area; and
cleaving the ground layer thereby forming a cleavage plane of the crystal layers for a laser resonator.
In an aspect of the fabrication method according to the invention, the wavelength of said light beam is selected from wavelengths passing through the substrate and absorbed by the ground layer in the vicinity of the interface.
In another aspect of the fabrication method according to the invention, the method further comprises, between said step of forming the crystal layers and said step of applying the light beam toward the interface, a step of bonding a cleavable second substrate onto a surface of the crystal layers in such a manner that a cleavage plane of the second substrate substantially coincides with a cleavage plane of the crystal layers of the nitride semiconductor.
As to a further aspect of the fabrication method according to the invention, in the step of applying the light beam toward the interface, the light beam is applied uniformly or entirely onto the interface between the substrate and the ground layer.
As to a still further aspect of the fabrication method according to the invention, in the step of applying the light beam toward the interface, the interface between the substrate and the ground layer is scanned with a spot or line of the light beam.
In another aspect of the fabrication method according to the invention, the method further comprises a step of forming a waveguide extending along a direction normal to the cleavage plane of the nitride semiconductor.
In a further aspect of the fabrication method according to the invention, the crystal layers of the nitride semiconductor are formed by metal-organic chemical vapor deposition.
As to a still further aspect of the fabrication method according to the invention, in the step of applying the light beam toward the interface, the light beam is an ultraviolet ray generated from a frequency quadrupled YAG laser.
In addition, a nitride semiconductor laser device according to the present invention having successively grown crystal layers each made of a group III nitride semiconductor (AlxGa1xe2x88x92x)1xe2x88x92YInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) comprises:
a ground layer made of group III nitride semiconductor (Alxxe2x80x2Ga1xe2x88x92xxe2x80x2)1xe2x88x92Yxe2x80x2Inyxe2x80x2N (0xe2x89xa6xxe2x80x2xe2x89xa61, 0xe2x89xa6yxe2x80x2xe2x89xa61);
a plurality of crystal layers each made of group III nitride semiconductor formed on the ground layer;
a cleavable substrate bonded onto a surface of the crystal layers opposite to the ground layer.
In another aspect of the nitride semiconductor laser device according to the invention, the device further comprises a heat sink bonded onto the ground layer.
In a further aspect of the nitride semiconductor laser device according to the invention, the device further comprises a heat sink bonded onto the cleavable substrate.
In a still further aspect of the nitride semiconductor laser device according to the invention, the cleavable substrate has a cleavage plane coinciding with a cleavage plane of the crystal layers of the nitride semiconductor.
In another aspect of the nitride semiconductor laser device according to the invention, the device further comprises a waveguide extending along a direction normal to the cleavage plane of the nitride semiconductor.
In a further aspect of the nitride semiconductor laser device according to the invention, the cleavable substrate is made of semiconductor single-crystal such as GaAs.
In a still further aspect of the nitride semiconductor laser device according to the invention, the cleavable substrate is made of an electrically conductive material.
According to the present invention, it is possible to obtain high-quality mirror facets by untying the crystal bond between the sapphire substrate and the ground layer of GaN crystal and separating the substrate and the ground layer and thereby, fabricating the laser device with high reproducibility.